SDRAM, or synchronous dynamic random access memory, is a type of memory integrated circuit that waits for rising or falling edges of a timing reference signal before responding to control inputs. Typical examples of timing reference signals include clock signals and strobe signals. DDR SDRAM, or double-data-rate SDRAM, achieves greater bandwidth than ordinary SDRAM by transferring data on both the rising and falling edges of timing reference signals.
Many DDR SDRAMs that produce data also produce a data strobe, called “DQS” (“data query strobe”), to indicate that data is valid. The DQS is transmitted, along with data, from the memory controller to the DDR SDRAM during write operations and from the DDR SDRAM from to the memory controller during read operations. When driven by the memory controller, DQS is center-aligned with the write data. When driven by the memory, DQS is edge-aligned with the read data.
The timing for write operations is often defined in a specification. For example, in at least some DDR SDRAM specifications, the time tDQSS between a write command and the first corresponding rising edge of DQS is specified with a relatively wide range (from 75% to 125% of one clock cycle). The time tDQSS might be described as a window during which the specified DDR SDRAM “looks for” data on a data bus. Devices issuing a write command to such a DDR SDRAM are expected to drive DQS in such a way that the signal arrives at the DRAM pins at a clock edge, plus or minus 25% of one clock cycle.
Designing a memory controller that provides the write DQS within a timing window tDQSS can be complicated by the fact that the memory controller is desired to operate in many different system topologies. For example, relatively short, lightly loaded channels may lead a DQS to arrive too early; whereas relatively long, highly loaded channels may lead a DQS to arrive too late. In either case, the early or late DQS may violate the specification requirement for the timing window tDQSS, and potentially lead to an error. Other system variations, such as those that result from process variations and temperature and supply-voltage fluctuations, also affect signal propagation delays and therefore further complicate the task of maintaining the relative timing of the DQS and the write signal within the requisite window.
Meeting the DQS timing window tDQSS can be particularly daunting if the memory device does not include clock recovery circuitry to stabilize the device-side clock signal used to time the write signal. DDR DRAMs adapted for use in mobile devices often lack clock-recovery circuitry, which advantageously reduces standby power and standby-active transition latency. Unfortunately, these benefits come at the cost of increased write signal drift, leading to an increased probability of violating the tDQSS timing parameter.